284 HOW DO STARFISHES OPEN OYSTERS ? 



6. Venus 3-7 cm. long, 3 cm, broad. 



9.50 p.m., loaded with 1000 grams. 



10.15 „ open 2 mm. 



10.30 „ open 4 mm. 



11.30 ,, open 4 mm. 



7.0 a.m. next morning, found with adductor muscles ruptured, 



7. Venus 3-3 cm. long, 2-8 cm, broad. 



8.0 a.m., loaded with 900 grams. 



8.24 „ open 1-5 mm. 



9.45 „ open 2-5 mm. 



10.20 „ open 3-5 mm. : then set free. 



Several of the molluscs closed somewhat when approached or 

 disturbed; but as long as the strain was continued they could never 

 shut up completely, even when their soft parts were mechanically 

 irritated. With the exception of the two which were torn apart, they 

 all closed again completely and tightly immediately they were freed 

 from the strain, and when left to themselves behaved quite normally. 



It will be seen from the tables that different individuals resist the 

 strain to a very different extent. Generally a weight of 900 grams* is 

 quite sufficient to open a Venus in from five to twenty-five minutes, or, 

 on an average, fifteen minutes. Further, it follows from experiments 3 

 and 6 that a strain of 2000, or even 1000 grams, exerted on both shells 

 at once, is sufficient, if continued for some time, to rupture the adductors; 

 whilst, according to the results both of Lawrence-Hamilton's experiments 

 and of my own, not even a weight of 4000 grams is enough to bring 

 about a sudden rupture. The difference in effect between a momentary 

 and a continuous strain is thus most clearly shown. 



We saw above that a starfish of moderate size can develop a force of 

 at least 1350 grams with the tube-feet which come into play ; so that 

 it possesses more than sufficient strength to forcibly open a Venus, 

 since for this purpose at most 900 grams is necessary. The conclusions 

 we have come to are therefore completely confirmed by experiment. 



What applies to a Venus applies also to an oyster ; which, according 

 to Lawrence-Hamilton, can only resist 1919*5 times its own weight 

 (without the shell), and hence is somewhat weaker than Venus. 



It hardly requires to be stated that every starfish cannot open every 

 oyster or bivalve, and that the size and strength of the two must be in 

 suitable proportion. 



* I have not ascertained how small a load is necessary to cause a slight opening of the 

 shell. It only concerned me to learn whether a weight of 1350 grams is large enough. 



